This invention relates to all BICMOS voltage controlled oscillators used in frequency synthesis phase-locked loop applications.
A voltage controlled oscillator (VCO) is a device whose output frequency (Fout) is a function of its input control voltage (Vcnt). The VCO is often used to track a reference frequency (Fref) of a reference frequency signal by the use a phase-locked loop (PLL). A phase detector detects a phase difference between the phase of the reference frequency signal and the phase of a divided frequency Fvco/N (N=1, 2, 3, 4, . . . ) of the VCO. Depending on this phase difference, an input node of the VCO is either charged or discharged, thereby altering the VCO's frequency of oscillation. When the frequencies of the two input signals of the phase detector are the same within a given error margin, the PLL is then called to be "locked" to the reference frequency signal. VCO design is critical in determining the overall performance of the PLL. The VCO's stability to small fluctuations in its input voltage is very important in determining PLL stability and jitter. The jitter can be thought of as spurious variations in the time positions of a series of successive pulses.
As will later be described in detail, in order to realize low jitter irrespective of process variations, the VCO which has multiple current ranges and which can choose the optimum current range for each process chip, is required. Manual current switching is troublesome for the customer and increases pin count while laser trimming is extremely expensive. Thus, an automatic device of providing each type of process chip with its optimum current range is important. It is also important that the lock can be maintained over a very wide operating temperature range.